The present invention refers to a device for jetting droplets of a viscous medium onto a substrate, preferably an electronic circuit board, said device comprising an eject chamber for containing a small volume of said medium prior to the jetting thereof, an eject nozzle communicating with the eject chamber, eject means for jetting said medium from the eject chamber through the eject nozzle, and supply means for feeding said medium into the eject chamber.
The invention also refers to a corresponding method for jetting droplets of a viscous medium onto a substrate, preferably an electronic circuit board.
Devices of the above mentioned kind are previously known which are primarily intended to be used for jetting small droplets of a solder paste, an electrically conductive glue, a surface mounting glue, or the like, on electric circuit boards before placing components intended to be surface mounted thereon, but which may be used also for applying other viscous mediums on any suitable kind of substrate.
For example, WO 91/12106 describes a method for rapidly dispensing droplets of a material such as solder paste and glue onto an electronic circuit board, wherein said material is fed from a container through an inlet, using overpressure, to a pump chamber and is then jetted through a nozzle connected to said chamber by the rapid reduction of the volume of the chamber using a magnetostrictive element.
A disadvantage with this kind of technique for providing the material from the reservoir to the chamber is that the feeding of the material is difficult to control due to, inter alia, the large distance between the pressure source and the chamber and the compressibility of the material.
An object of the invention is to provide a solution which allows for a simple, reliable and more accurate control of the supply of a viscous medium to an eject chamber.
This and other objects are achieved by the invention as defined in the accompanying claims.
Hence, according to a first aspect of the invention, there is provided a device of the kind mentioned in the introduction, said device being characterised by said supply means comprising: means defining a tubular passage; an inlet for introducing said medium into said tubular passage and an outlet for passing said medium from said tubular passage towards said chamber; a rotatable feed screw provided in said tubular passage for, upon rotation thereof, forcing said medium through said tubular passage in the direction towards said chamber; and drive means for rotating said rotatable feed screw.
According to a second aspect of the invention, there is provided a method of the kind mentioned in the introduction, said method being characterised by the steps of: supplying said medium from a viscous medium supply container to an eject chamber using a rotatable feed screw; and jetting said medium from the eject chamber in the form of droplets.
It is to be noted that the eject means, as used in this application, refers to the means that interacts in the rapid reduction of the eject chamber for providing the jetting of droplets of said viscous medium. The jetting of droplets is achieved completely separate from the rotational movement of the feed screw, the rotational movement providing the feeding of said medium to the eject chamber.
For the purposes of this application, it is furthermore to be noted that the term xe2x80x9cviscous mediumxe2x80x9d should be interpreted as solder paste, flux, adhesive, conductive adhesive, or any other kind of medium used for fastening components on a substrate, or resistive paste; and that the term xe2x80x9csubstratexe2x80x9d should be interpreted as a printed circuit board (PCB), a substrate for ball grid arrays (BGA), chip scale packages (CSP), quad flat packages (QFP), and flip-chips or the like. It is also to be noted that the term xe2x80x9cjettingxe2x80x9d should be interpreted as a non-contact dispensing process that utilises a fluid jet to form and shoot droplets of a viscous medium from a jet nozzle onto a substrate, as compared to a contact dispensing process, such as xe2x80x9cfluid wettingxe2x80x9d, which is the act of the viscous medium leaving the dispense tip, contacting and clinging to the substrate and remaining on the substrate as the dispense tip pulls away.
Thus, the invention is based upon the idea of using a rotatable feed screw for feeding the viscous medium to the eject chamber. The use of such a rotatable feed screw makes it possible to control the feeding of viscous medium to the eject chamber in an advantageously accurate, rapid and simple manner. As is understood, rotational movement of the rotatable feed screw will immediately affect the behaviour of the viscous medium at the output end of the rotatable feed screw near the eject chamber, whereas control of a pressure device according to prior art will cause an effect at the eject chamber only after the passing of a certain time delay which is given by the time it takes for a pressure wave to travel through the medium from the pressure device to the chamber. The use of a rotatable feed screw even makes it possible to partly withdraw viscous medium from the eject chamber by reversing the direction of rotation of the feed screw. According to the invention, the volume in which the compressibility of the viscous medium may have a negative effect is decreased considerably, thus limiting the negative effects caused by said compressibility.
In this context, it is noted that the use of a rotatable screw is known per se, for example as disclosed in WO 97/19327 as an augering screw and in U.S. Pat. No. 5,564,606 as a feed screw, for accomplishing dispensing of solder paste onto an electronic circuit board using so called xe2x80x9cfluid wettingxe2x80x9d, as described above, wherein the viscous medium makes contact with the electronic circuit board while the medium is still connected to the nozzle. Such xe2x80x9cfluid wettingxe2x80x9d is not designed for the jetting, as defined above, or even for the rapid dispensing, of solder paste onto an electronic circuit board.
According to a preferred embodiment of the invention, at least an inner portion of said means defining a tubular passage shows elastic, or resilient, properties. One difficulty in using a rotatable feed screw to transfer, e.g., a solder paste is that the spheres or particles of solder material dispersed in the solder paste may be negatively smeared out, or the like, by the action of the threads of the rotational feed screw against the surrounding tubular passage. It has surprisingly been found that such negative effects may be reduced or even eliminated if the walls surrounding the rotatable feed screw, and generally making sliding contact with the threads thereof, is made resilient. According to one embodiment, this is accomplished by the provision of an elastomeric tubular sleeve surrounding at least a portion of said rotatable feed screw. According to another alternative, an array of elastomeric o-rings surrounding at least a portion of said rotatable feed screw is provided to form said tubular passage. As is understood, this preferred embodiment and these alternatives are especially preferred when the invention is used for applying solder paste. Furthermore, providing a rotational feed screw with said features according to this particular preferred embodiment could advantageosly also be used in a dispensing device of a general kind for feeding a viscous medium to a dispensing location in the dispensing device for subsequent dispensing from the dispensing location onto a suitable substrate. Making said surrounding walls resilient also has the advantageous effect of allowing greater manufacturing tolerances.
Typically, said medium will be provided from a viscous medium supply container communicating with said inlet to the rotatable feed screw for providing a source for said medium. According to a preferred embodiment, a pressure is applied onto the medium supplied from said supply container, said pressure being sufficiently large to ensure an essentially void free condition within said tubular passage and being sufficiently small not to force said medium through said tubular passage in excess of the feed action provided by said rotatable feed screw.
As will be described below with reference to the accompanying drawings, according to one embodiment, said tubular passage and said rotatable feed screw are arranged coaxially with respect to the direction of movement of said eject means, and, according to another embodiment, said tubular passage and said rotatable feed screw are arranged to supply said medium at an angle with respect to the direction of movement of said eject means. Advantages of the former alternative is that a more compact, integrated and less bulky arrangement is typically achieved and that it makes it easier to provide sufficient sealing, whereas an advantage of the latter alternative is that a more separated system is typically achieved, often making it easier to dismantle and clean. Also, in the former alternative, an end portion of the rotatable feed screw may preferably define a plunger forming a wall of said eject chamber.
According to another preferred embodiment of the invention, said medium is fed from the output end of said tubular passage to said eject chamber through a channel provided within a plunger defining a wall of said eject chamber. Preferably, at least a portion of said channel at the end of the plunger facing said eject chamber is coaxially arranged with respect to said plunger. When rapidly decreasing the volume of the eject chamber, the medium contained therein is both ejected through the eject nozzle and forced backwards towards the rotational feed screw. These opposite actions will depend upon, inter alia, the flow resistance in the backwards direction. If the medium is directed to said eject chamber laterally with respect to said plunger, this flow resistance will vary as the distance between the plunger and the nozzle is changed, thus placing high demands upon the design tolerances of this distance. According to said embodiment, wherein a channel leading to the eject chamber is provided within the plunger, the flow resistance in the backwards direction will be independent of the distance between the plunger and the nozzle, thus making it easier to control the eject operation.
It is understood that the rotatable feed screw of the invention may be formed in many different ways. However, preferable results have been achieved with the use of a double-threaded screw.
Furthermore, the feed screw according to a first embodiment of the invention constitutes a constructional element which is separate from said eject means. According to a second embodiment of the invention, the feed screw is also used for the ejection of the viscous medium, i.e. comprised in the eject means, for instance as a movable eject element, or as a rigid element providing a counter support for said ejection. However, it must to be noted that even though the feed screw is involved in the ejection of the viscous medium, the rotational movement of the feed screw is not.
As will be described below with reference to the drawings, the medium is preferably jetted from said eject nozzle by the rapid actuation of a magnetostrictive, piezoelectric, electrostrictive, electromagnetic, or shape memory alloy actuator.
The above mentioned and other aspects, advantages, and features of the invention will be more fully understood from the following description of exemplifying embodiments thereof.